1. Field of the Invention
The present invention relates generally to semiconductor pickup devices and particularly to those having a plurality, of pixel circuits arranged in a matrix.
2. Description of the Background Art
CCDs, CMOS image sensors and similar semiconductor pickup devices (also referred to as solid state image sensing devices, semiconductor image sensors) have now been mounted in video cameras, digital cameras as well as mobile phones and thus widely used in our daily life. In recent years semiconductor pickup devices are remarkable improved in performance, and wide dynamic range technology has also helped to develop that allowing a pickup allowing illuminance range of six digits or more.
FIG. 14 is a circuit diagram showing a configuration of a pixel circuit of such a semiconductor pickup device. In the figure this pixel circuit includes a photodiode 30, a log transistor 31, a reset transistor 32, and a read transistor 33. Transistors 30-33 are each implemented by an n channel MOS transistor. Photodiode 30 has an anode connected to a line of a ground potential GND and a cathode to a node N30. Log transistor 31 has its gate and drain connected to a line of a power supply potential VDD and its source to node N30. Reset transistor 32 has its drain receiving a reset potential VRH, its gate receiving a reset signal RST, and its source connected to node N30. Read transistor 33 has its gate connected to node N30.
When reset signal RST is set in a pulse to a high level of an active level, reset transistor 32 conducts and node N30 is reset to reset potential VRH. When photodiode 30 receives light, a current IP of a value corresponding to the received light's intensity flows from node N30 through photodiode 30 to the line of ground potential GND, and also from the line of power supply potential VDD through log transistor 31 to node N30 flows a current of a value corresponding to VDS-IDS characteristic of log transistor 31. A current IDS of log transistor 31 presents a logarithmical response for a range high in potential VDS, and if the received light has large intensity the current of read transistor 33 nonetheless hardly saturates. Thus the pixel circuit allows detection of a wide range of intensity of received light.
A semiconductor pickup device has a large number of such pixel circuits arranged in a matrix. Each pixel circuit receives light, which is detected in intensity by read transistor 33 to pick up an image of an object (see U.S. Pat. No. 6,323,479 for example).
For such a semiconductor image pickup device, however, all pixel circuits' frame rates are set to have the same value. As such, if for a frame rate (for example of 30 frames/second) a satisfactory image is obtained for the subject's lower illuminance area as well as higher illuminance area, for increased frame rates, however, minimum illuminance required for pickup is reduced, resulting in a blackout image for the lower illuminance area. In other words, if a semiconductor image pickup device having a wide dynamic range can be used to ensure a bandwidth of illuminance allowing an image to be picked up, minimum illuminance required for pickup cannot be ensured. This is not simply a problem associated only with high speed cameras or similar, specific applications but also all semiconductor image pickup device applications as the number of pixels tends to increase in recent years.